CSC 1051 M.A. Papalaskari, Villanova University Everyday objects: Strings and Wrappers CSC 1051 –...

transcript

CSC 1051 M.A. Papalaskari, Villanova University

Everyday objects: Strings and Wrappers

CSC 1051 – Data Structures and Algorithms IDr. Mary-Angela Papalaskari

Department of Computing Sciences

Villanova University

Course website:

http://www.csc.villanova.edu/~map/1051/f13

Some slides in this presentation are adapted from the slides accompanying Java Software Solutions by Lewis & Loftus

Overview• Review what we know about objects:

– classes– methods– object creation

• String class– useful methods and examples

• Wrapper classes

Some everyday Objects…

• Strings - defined by the String class:

"This is a string literal.""123 Main Street""X”

• System.out is also an object - it represents a destination (the monitor screen) to which we can send output

Methods• Objects can have methods associated with them

• In Lincoln.java we invoked the println method

System.out.println ("Whatever you are, be a good one.");

object methodname

information provided to the method(parameters)

Invoking Methods• We use the dot operator to invoke an object’s methods

int numOfCharsInName = name.length();

length() is one of the methods of String objects (defined in String class)

String name = scan.nextLine();

nextLine() is one of the methods of Scanner objects (defined in Scanner class)

B yste

0 1 2 3 4

More String Methods

int numOfCharsInName = name.length();

char initial = name.charAt(0);

B yste

0 1 2 3 4

String newName = name.replace('s', 't');

String capsName = name.toUpperCase();

String nickName = name.substring(1, 4);

0 1 2 3 4

newName

0 1 2 3 4

capsName

0 1 2 3 4

nickNameSee also textbook example StringMutation.java

Palindrome tester• Input a string, determine whether it is a palindrome,

i.e.:– first char is the same as last char– 2nd char is the same as 2nd last char– and so on…

• How to express this as an algorithm?• How to implement it?

R RADA

0 1 2 3 4str

System.out.println ("Enter a potential palindrome:");str = scan.nextLine();

left = 0;right = str.length() - 1;

while (str.charAt(left) == str.charAt(right) && left < right){ left++; right--;}

if (left < right) System.out.println ("NOT a palindrome");else System.out.println ("palindrome");

PalindromeTester.java (Example from Chapter 5)

System.out.println ("Enter a potential palindrome:");str = scan.nextLine();

left = 0;right = str.length() - 1;

while (str.charAt(left) == str.charAt(right) && left < right){ left++; right--;}

if (left < right) System.out.println ("NOT a palindrome");else System.out.println ("palindrome");

PalindromeTester.java (Example from Chapter 5)

Sample RunEnter a potential palindrome:radarpalindrome

Test another palindrome (y/n)? yEnter a potential palindrome:able was I ere I saw elbapalindrome.

Test another palindrome (y/n)? yEnter a potential palindrome:abracadabraNOT a palindrome.

Test another palindrome (y/n)? n

Declaring Variables, revisited• Examples of variable declarations:

int count = 0;

double mpg;

String title;

Graphics page;

Color aquamarine;

Scanner scan;

• A class name can be used as a type to declare an object reference variable

• The object itself must be created separatelyCSC 1051 M.A. Papalaskari, Villanova University

Creating Objects • We have already seen something like this:

Scanner scan = new Scanner (System.in);

The new operator calls the Scanner constructor, which isa special method that sets up the object

Variable refers to a Scanner object Constructing a new object is called instantiation

an instance of the Scanner class

Creating Objects • Another example:

The new operator calls the String constructor, which isa special method that sets up the object

Variable refers to a String object Constructing a new object is called instantiation

an instance of the String class

String title = new String ("Java Software Solutions");

The String Class is SPECIAL!

• Exception to the use of new operator: Because strings are so common, we don't have to use the new operator to create a String object

• This is special syntax that works only for strings

String title = new String ("Java Software Solutions");

String title = "Java Software Solutions";

Wrapper Classes• The java.lang package contains wrapper

classes that correspond to each primitive type:

Primitive Type Wrapper Classbyte Byte

short Short

int Integer

long Long

float Float

double Double

char Character

boolean Boolean

Wrapper Classes• The following declaration creates an Integer

object which represents the integer 40 as an object

Integer age = new Integer(25);

Wrapper Class methods and constants• Integer.parseInt(): convert String to int

• Double.parseDouble(): convert String to double

• Integer.MIN_VALUE: smallest int value

• Integer. MAX_VALUE: largest int value

Examples:

String str = scan.nextLine();

int num = Integer.parseInt(str);

int currentMin = Integer.MAX_VALUE;

Autoboxing• Autoboxing is the automatic conversion of a

primitive value to a corresponding wrapper object:

Integer obj;int num = 42;obj = num;

• The assignment creates the appropriate Integer object

• The reverse conversion (called unboxing) also occurs automatically as needed

Quick Check

Are the following assignments valid?

Double value = 15.75;

Character ch = new Character('T');

char myChar = ch;

References• Note that a primitive variable contains the value

itself, but an object variable contains the address of the object

• An object reference can be thought of as a pointer to the location of the object

• Rather than dealing with arbitrary addresses, we often depict a reference graphically

"Steve Jobs"name1

num1 38

Assignment Revisited• The act of assignment takes a copy of a value and

stores it in a variable

• For primitive types:

num1 38

num2 96Before:

num2 = num1;

num1 38

num2 38After:

int num1 = 38;int num2 = 96;

Reference Assignment• For objects, the same is true, but what is copied is

the reference to the object (i.e., its address):

• For objects:

num2After:

num2Before:

num2 = num1;

Integer num1 = 38;Integet num2 = 96;

Another example• For object references, assignment copies the

address:

name2 = name1;

name2Before:

"Steve Jobs"

"Steve Wozniak"

name2After:

"Steve Jobs"

Aliases• Two or more references that refer to the same

object are called aliases of each other

• That creates an interesting situation: one object can be accessed using multiple reference variables

• Aliases can be useful, but should be managed carefully

• Changing an object through one reference changes it for all of its aliases, because there is really only one object

Garbage Collection• When an object no longer has any valid references

to it, it can no longer be accessed by the program

• The object is useless, and therefore is called garbage

• Java performs automatic garbage collection periodically, returning an object's memory to the system for future use

• In other languages, the programmer is responsible for performing garbage collection

CSC 1051 M.A. Papalaskari, Villanova University Everyday objects: Strings and Wrappers CSC 1051 –...

Documents